1. Field of the Invention
The present invention relates to an image forming method used in an image forming apparatus such as a copier, a facsimile, a printer, etc.; a toner used in an image forming apparatus; and an image forming apparatus using the toner.
2. Description of the Related Art
As a toner supply device that supplies toner in a toner storage container to a developer housing section of a developing device in an image forming apparatus, there has conventionally been a toner supply device in which toner is conveyed using a screw pump, as described in Japanese Patent (JP-B) No. 3872506 or Japanese Patent Application Laid-Open (JP-A) No. 2007-079504. Such a toner supply device is equipped with a conveyance path member through which toner passes, and a screw pump; toner in the toner storage container is conveyed to the developer housing section by the suction of the screw pump.
The screw pump is provided with a stator which is a cylindrical elastic member having a spiral groove in its inner wall surface, and a rotor which is a spiral metal member that rotates inside the stator. Between the stator and the rotor, there is formed an enclosed space surrounded by their surfaces and contact portions where they are in contact with each other; as the rotor rotates, it slides and rubs on the stator, and this rotation causes the portion of the rotor in contact with the stator to move, thereby making the enclosed space move in the rotational axis direction of the rotor. The screw pump is provided with opening portions at both its ends with respect to the rotational axis direction of the rotor, and the opening portion on the upstream side in the moving direction of the enclosed space serves as a suction opening, whereas the opening portion on the downstream side in the moving direction of the enclosed space serves as an outlet. The suction opening is connected to the toner storage container via a conveyance tube that is a conveyance path member, and the outlet is connected to the developer housing section of the developing device directly or via a conveyance path member or a hopper member.
In the toner supply device equipped with the screw pump, when the screw pump is driven, negative pressure is generated at the suction opening due to the movement of the enclosed space caused by the rotation of the rotor, and toner in the toner storage container is sucked and thusly enters between the stator and the rotor of the screw pump. The toner having entered the screw pump is enclosed between the stator and the rotor due to the movement of the contact portions of the stator and the rotor caused by the rotation of the rotor, then the toner is conveyed to the outlet by the movement of the enclosed space. The toner having been discharged from the outlet of the screw pump is supplied to the developer housing section directly or via the conveyance path member or the hopper member.
In the foregoing toner supply device equipped with the screw pump, since toner is conveyed by the suction of the screw pump, it is possible to convey the toner even if the conveyance tube is curved at a steep angle or ascends at a steep angle, for instance. Therefore, use of the screw pump makes it possible to increase the degree of freedom of the layout of the toner supply device. Also, it is possible to place a conveyance tube between components in the vicinity of the toner supply device and thus to make an image forming apparatus compact.
In the screw pump, however, since toner is conveyed with the contact portion of the rotor and the contact portion of the stator rubbing against each other, the toner is given stress attributable to frictional heat, shearing force, pressure, etc. in gaps between the rotor and the stator, which exist at the contact portions, and thus toner aggregates of 0.1 mm to 1 mm in diameter are produced in some cases. The cohesion of these aggregates is weak to such an extent that they crumble when touched by fingertips; thus, some of the aggregates are broken into fine pieces through agitation by an agitating member inside the developer housing section and come back into the state of ordinary toner powder even if supplied to the developing device along with other toner. In some cases, however, some others of the aggregates with higher cohesiveness are not broken into fine pieces inside the developer housing section and thusly supplied to a developing roller that supplies toner to a latent image on the surface of a photoconductor. Once the aggregates supplied to the developing roller have been supplied onto the photoconductor surface, they can no longer be broken into fine pieces. When these aggregates are transferred from the photoconductor to a recording medium such as transfer paper by a transfer device, image defect is caused. For instance, in the case of a solid image (in which a single color is used and the amount of toner attached is 0.45 mg/cm2), an abnormal image (hereinafter referred to as “firefly”) is produced in which a deep color spot is formed at the part where the aggregates have been attached, and a pale color area is formed in the vicinity of the deep color spot. Meanwhile, in the case of a linear image, the part where the aggregates have been attached blackens, and thus thin lines may be unable to be reproduced.
To prevent the image defect which stems from the formation of aggregates, it is required that toner particles not easily adhere to one another by electrostatic or nonelectrostatic adhesion and that the cohesiveness of the toner particles not exceed a predetermined level, in other words constant fluidity of the toner particles be maintained, even after stress attributable to heat, pressure, etc. has been applied to the toner particles inside the screw pump.
In maintaining constant fluidity of toner particles, the degree of circularity of the toner particles and the amount of an external additive, which affect the fluidity of the toner particles, are important factors. The closer the degree of circularity of toner particles is to 1.0, in other words the closer they are to spheres in shape, the higher their fluidity is. Conversely, the more the degree of circularity of toner particles deviates from 1.0, in other words the more they deform, the lower their fluidity is. Meanwhile, as for fine silica particles (hereinafter referred to as “small particle size silica”) having a BET specific surface area of approximately 50 m2/g to 400 m2/g commonly used as an external additive for toner, the larger their amount is, the higher the fluidity of toner particles is. Conversely, the smaller their amount is, the lower the fluidity of toner particles is. This is because nonelectrostatic adhesion between the toner particles can be reduced to a greater extent, as the degree of circularity of the toner particles is made closer to 1.0 or the fine silica particles are added in larger amounts. Accordingly, by making toner particles closer to spheres in shape or increasing the amount of small particle size silica added, it is possible to enhance the fluidity of the toner particles.
However, when the small particle size silica is excessively added, the following troubles arise in some cases. When the small particle size silica is added in larger amounts than necessary to toner base particles having a high degree of circularity, such a trouble is caused that there is a tremendous increase in fluidity and thus an excessive increase in the bulk density of the toner. There is a device for optimizing the toner concentration in a developer by means of a judgment based upon the bulkiness of the developer, made by a sensor provided in a developing device. In the foregoing device, when the bulk density of the developer is excessively high, the toner concentration is most suitable, but the sensor is likely to judge that the concentration of developer components other than the toner is higher than the toner concentration, and so the toner may be excessively supplied, thereby possibly leading to an extremely high toner concentration as a result.
Meanwhile, when an attempt is made to rectify the poor fluidity of toner particles having a low degree of circularity by merely adding the small particle size silica, it needs to be added in large amounts, and thus the proportion of the small particle size silica to the toner base particles becomes high. As the proportion of the small particle size silica to the toner base particles becomes high, the toner-fixing property degrades. Moreover, in the case where a cleaning device for removing toner on the surface of a toner image bearing member such as a photoconductor removes the toner by means of a blade, the blade is often partially abraded if a high proportion of small particle size silica is contained in the toner to be removed. When the blade of the cleaning device is partially abraded, such a cleaning defect is caused in which toner remains in the form of a streak on a part of the toner image bearing member surface facing the abraded part of the blade, and this cleaning defect leads to an image defect in which a black streak appears in an image formed on a recording medium. Prevention of the occurrence of such troubles caused by excessive addition of small particle size silica necessitates optimizing the degree of circularity of toner base particles and the amount of small particle size silica added.